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                <ol class="chapter"><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">1.</strong> 类型系统</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../01_类型系统/Rust_Concept_Clarification_Deref_vs_AsRef_vs_Borrow_vs_Cow/Rust_Concept_Clarification_Deref_vs_AsRef_vs_Borrow_vs_Cow.html"><strong aria-hidden="true">1.1.</strong> Rust Concept Clarification Deref vs AsRef vs Borrow vs Cow</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/Rust_Concept_Clarification_Deref_vs_AsRef_vs_Borrow_vs_Cow/Deref_AsRef_Borrow_Cow释义.html"><strong aria-hidden="true">1.2.</strong> Deref AsRef Borrow Cow 释义</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/Rust的Borrow和AsRef：让你的代码用起来像呼吸一样自然/Rust的Borrow和AsRef：让你的代码用起来像呼吸一样自然.html"><strong aria-hidden="true">1.3.</strong> Rust的Borrow和AsRef：让你的代码用起来像呼吸一样自然</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/Rust的Cow类型有什么用？详解Cow及其用途/Rust的Cow类型有什么用？详解Cow及其用途.html"><strong aria-hidden="true">1.4.</strong> Rust的Cow类型有什么用？详解Cow及其用途</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/判别Fn、FnMut、FnOnce的标准/判别Fn、FnMut、FnOnce的标准.html"><strong aria-hidden="true">1.5.</strong> 判别Fn、FnMut、FnOnce的标准</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/一行代码告诉你内部可变性的真相(UnsafeCell)/一行代码告诉你内部可变性的真相(UnsafeCell).html"><strong aria-hidden="true">1.6.</strong> 一行代码告诉你内部可变性的真相(UnsafeCell)</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/Tour_of_Rust's_Standard_Library_Traits/Tour_of_Rust's_Standard_Library_Traits.html"><strong aria-hidden="true">1.7.</strong> Tour of Rust's Standard Library Traits</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/逆变、协变与子类型，以及Rust/逆变、协变与子类型，以及Rust.html"><strong aria-hidden="true">1.8.</strong> 逆变、协变与子类型，以及Rust</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/Rust自引用结构、Pin与Unpin/Rust自引用结构、Pin与Unpin.html"><strong aria-hidden="true">1.9.</strong> Rust自引用结构、Pin与Unpin</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/为什么Rust需要Pin,Unpin/为什么Rust需要Pin,Unpin.html"><strong aria-hidden="true">1.10.</strong> 译：为什么 Rust 需要 Pin, Unpin ？</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/定海神针Pin和Unpin/定海神针Pin和Unpin.html"><strong aria-hidden="true">1.11.</strong> 译：定海神针 Pin 和 Unpin</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/sizedness-in-rust/sizedness-in-rust.html"><strong aria-hidden="true">1.12.</strong> Sizedness in Rust</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/Rust生命周期集大成者PhantomData〈T〉/Rust生命周期集大成者PhantomData〈T〉.html"><strong aria-hidden="true">1.13.</strong> Rust生命周期集大成者 PhantomData&lt;T&gt;</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/数据库表达式执行的黑魔法：用Rust做类型体操/数据库表达式执行的黑魔法：用Rust做类型体操_Part_0.html"><strong aria-hidden="true">1.14.</strong> 数据库表达式执行的黑魔法：用Rust做类型体操 Part 0</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/数据库表达式执行的黑魔法：用Rust做类型体操/数据库表达式执行的黑魔法：GAT实现引用类型关联_Part_1.html"><strong aria-hidden="true">1.15.</strong> 数据库表达式执行的黑魔法：GAT实现引用类型关联 Part 1</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/数据库表达式执行的黑魔法：用Rust做类型体操/数据库表达式执行的黑魔法：用HRTB写bound_Part_2.html"><strong aria-hidden="true">1.16.</strong> 数据库表达式执行的黑魔法：用HRTB写bound Part 2</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/数据库表达式执行的黑魔法：用Rust做类型体操/数据库表达式执行的黑魔法：用Rust做类型体操之用宏展开重复代码_Part_3_&_4.html" class="active"><strong aria-hidden="true">1.17.</strong> 数据库表达式执行的黑魔法：用Rust做类型体操之用宏展开重复代码 Part 3 & 4</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/数据库表达式执行的黑魔法：用Rust做类型体操/数据库表达式执行的黑魔法：与Rust编译器斗智斗勇之表达式向量化_Part_5_&_6.html"><strong aria-hidden="true">1.18.</strong> 数据库表达式执行的黑魔法：与Rust编译器斗智斗勇之表达式向量化 Part 5 & 6</a></li><li class="chapter-item expanded "><a href="../../01_类型系统/数据库表达式执行的黑魔法：用Rust做类型体操/数据库表达式执行的黑魔法：在Rust中用宏关联逻辑类型和实际类型_Part_7.html"><strong aria-hidden="true">1.19.</strong> 数据库表达式执行的黑魔法：在Rust中用宏关联逻辑类型和实际类型 Part 7</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">2.</strong> 生命周期</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../02_生命周期/Rust中的生命周期——从StrSplit实例说开去/Rust中的生命周期——从StrSplit实例说开去.html"><strong aria-hidden="true">2.1.</strong> Rust中的生命周期——从StrSplit实例说开去</a></li><li class="chapter-item expanded "><a href="../../02_生命周期/与ChatGPT深度对话来学Rust生命周期/与ChatGPT深度对话来学Rust生命周期.html"><strong aria-hidden="true">2.2.</strong> 与ChatGPT深度对话来学Rust生命周期</a></li><li class="chapter-item expanded "><a href="../../02_生命周期/进击的Rust生命周期——early_bound与late_bound（1）/进击的Rust生命周期——early_bound与late_bound（1）.html"><strong aria-hidden="true">2.3.</strong> 进击的Rust生命周期——early_bound与late_bound（1）</a></li><li class="chapter-item expanded "><a href="../../02_生命周期/进击的Rust生命周期——early_bound与late_bound（2）/进击的Rust生命周期——early_bound与late_bound（2）.html"><strong aria-hidden="true">2.4.</strong> 进击的Rust生命周期——early_bound与late_bound（2）</a></li><li class="chapter-item expanded "><a href="../../02_生命周期/进击的Rust生命周期——一力降十会的MIR（1）/进击的Rust生命周期——一力降十会的MIR（1）.html"><strong aria-hidden="true">2.5.</strong> 进击的Rust生命周期——一力降十会的MIR（1）</a></li><li class="chapter-item expanded "><a href="../../02_生命周期/进击的Rust生命周期——一力降十会的MIR（2）/进击的Rust生命周期——一力降十会的MIR（2）.html"><strong aria-hidden="true">2.6.</strong> 进击的Rust生命周期——一力降十会的MIR（2）</a></li><li class="chapter-item expanded "><a href="../../02_生命周期/Common_Rust_Lifetime_Misconceptions/Common_Rust_Lifetime_Misconceptions.html"><strong aria-hidden="true">2.7.</strong> Common Rust Lifetime Misconceptions</a></li><li class="chapter-item expanded "><a href="../../02_生命周期/Rust生命周期常见误区/Rust生命周期常见误区.html"><strong aria-hidden="true">2.8.</strong> 译：Rust生命周期常见误区</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">3.</strong> 无畏并发</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../05_无畏并发/简单写个Rust无锁队列/简单写个Rust无锁队列.html"><strong aria-hidden="true">3.1.</strong> 简单写个Rust无锁队列</a></li><li class="chapter-item expanded "><a href="../../05_无畏并发/进击的Rust多线程——混合自旋锁/进击的Rust多线程——混合自旋锁.html"><strong aria-hidden="true">3.2.</strong> 进击的Rust多线程——混合自旋锁</a></li><li class="chapter-item expanded "><a href="../../05_无畏并发/An_unsafe_tour_of_Rust's_Send_and_Sync/An_unsafe_tour_of_Rust's_Send_and_Sync.html"><strong aria-hidden="true">3.3.</strong> An unsafe tour of Rust's Send and Sync</a></li><li class="chapter-item expanded "><a href="../../05_无畏并发/进击的Rust多线程——Send与Sync/进击的Rust多线程——Send与Sync.html"><strong aria-hidden="true">3.4.</strong> 进击的Rust多线程——Send与Sync</a></li><li class="chapter-item expanded "><a href="../../05_无畏并发/进击的Rust多线程——离经叛道的PhantomData/进击的Rust多线程——离经叛道的PhantomData.html"><strong aria-hidden="true">3.5.</strong> 进击的Rust多线程——离经叛道的PhantomData</a></li><li class="chapter-item expanded "><a href="../../05_无畏并发/Rust_Async_Pin概念解析/Rust_Async_Pin概念解析.html"><strong aria-hidden="true">3.6.</strong> Rust Async: Pin概念解析</a></li><li class="chapter-item expanded "><a href="../../05_无畏并发/Rust和C++的并发库对比/Rust和C++的并发库对比.html"><strong aria-hidden="true">3.7.</strong> 译：Rust 和 C++ 的并发库对比</a></li><li class="chapter-item expanded "><a href="../../05_无畏并发/Rust原子类型和内存排序/Rust原子类型和内存排序.html"><strong aria-hidden="true">3.8.</strong> Rust原子类型和内存排序</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">4.</strong> 网络编程</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../06_网络编程/从编解码层面理解WebSocket_手写一个WebSocket/从编解码层面理解WebSocket_手写一个WebSocket.html"><strong aria-hidden="true">4.1.</strong> 从编解码层面理解WebSocket 手写一 个WebSocket</a></li><li class="chapter-item expanded "><a href="../../06_网络编程/透过Rust探索系统的本原：网络篇/透过Rust探索系统的本原：网络篇.html"><strong aria-hidden="true">4.2.</strong> 透过Rust探索系统的本原：网络篇</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">5.</strong> 轮子系列</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../07_轮子系列/700行Rust写一个内存分配器/700行Rust写一个内存分配器.html"><strong aria-hidden="true">5.1.</strong> 700行Rust写一个内存分配器</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/Rust：网络库的实现思路/Rust：网络库的实现思路.html"><strong aria-hidden="true">5.2.</strong> Rust：网络库的实现思路</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/Rust异步运行时基础部件/Rust异步运行时基础部件.html"><strong aria-hidden="true">5.3.</strong> Rust异步运行时基础部件</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/使用Rust+epoll编写异步IO框架/使用Rust+epoll编写异步IO框架（1）.html"><strong aria-hidden="true">5.4.</strong> 使用Rust+epoll编写异步IO框架（1）</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/使用Rust+epoll编写异步IO框架/使用Rust+epoll编写异步IO框架（2）.html"><strong aria-hidden="true">5.5.</strong> 使用Rust+epoll编写异步IO框架（2）</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/使用Rust+epoll编写异步IO框架/使用Rust+epoll编写异步IO框架（3）.html"><strong aria-hidden="true">5.6.</strong> 使用Rust+epoll编写异步IO框架（3）</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/用rust从零开发一套web框架/用rust从零开发一套web框架：day1.html"><strong aria-hidden="true">5.7.</strong> 用rust从零开发一套web框架：day1</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/用rust从零开发一套web框架/用rust从零开发一套web框架：day2.html"><strong aria-hidden="true">5.8.</strong> 用rust从零开发一套web框架：day2</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/用rust从零开发一套web框架/用rust从零开发一套web框架：day3.html"><strong aria-hidden="true">5.9.</strong> 用rust从零开发一套web框架：day3</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/用rust从零开发一套web框架/用rust从零开发一套web框架：day4.html"><strong aria-hidden="true">5.10.</strong> 用rust从零开发一套web框架：day4</a></li><li class="chapter-item expanded "><a href="../../07_轮子系列/用rust从零开发一套web框架/用rust从零开发一套web框架：day5.html"><strong aria-hidden="true">5.11.</strong> 用rust从零开发一套web框架：day5</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">6.</strong> 奇技淫巧</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../08_奇技淫巧/Copy-On-Write是不是优化？/Copy-On-Write是不是优化？.html"><strong aria-hidden="true">6.1.</strong> 译：Copy-On-Write是不是优化？</a></li><li class="chapter-item expanded "><a href="../../08_奇技淫巧/揭秘神奇的Rust_Axum风格的函数实现/揭秘神奇的Rust_Axum风格的函数实现.html"><strong aria-hidden="true">6.2.</strong> 译：揭秘神奇的 Rust Axum 风格的函数实现</a></li><li class="chapter-item expanded "><a href="../../08_奇技淫巧/“变长参数”函数与回调/“变长参数”函数与回调.html"><strong aria-hidden="true">6.3.</strong> “变长参数”函数与回调</a></li><li class="chapter-item expanded "><a href="../../08_奇技淫巧/Rust字符串格式化的幕后：format_args!()/Rust字符串格式化的幕后：format_args!().html"><strong aria-hidden="true">6.4.</strong> 译：Rust字符串格式化的幕后：format_args!()</a></li><li class="chapter-item expanded "><a href="../../08_奇技淫巧/给Rust带来一点C++特产/给Rust带来一点C++特产.html"><strong aria-hidden="true">6.5.</strong> 给Rust带来一点C++特产</a></li><li class="chapter-item expanded "><a href="../../08_奇技淫巧/一步步实现_Rust_Bevy_ECS_的_System_简化版本/一步步实现_Rust_Bevy_ECS_的_System_简化版本.html"><strong aria-hidden="true">6.6.</strong> 一步步实现 Rust Bevy ECS 的 System 简化版本</a></li><li class="chapter-item expanded "><a href="../../08_奇技淫巧/Exploring_Design_Patterns_in_Rust_with_Algorithmic_Trading_Examples/Exploring_Design_Patterns_in_Rust_with_Algorithmic_Trading_Examples.html"><strong aria-hidden="true">6.7.</strong> Exploring Design Patterns in Rust with Algorithmic Trading Examples</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">7.</strong> 源码分析</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../09_源码分析/Rust并发：bytes源码分析/Rust并发：bytes源码分析.html"><strong aria-hidden="true">7.1.</strong> Rust并发：bytes源码分析</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Rust并发：标准库Arc源码分析/Rust并发：标准库Arc源码分析.html"><strong aria-hidden="true">7.2.</strong> Rust并发：标准库Arc源码分析</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Rust并发：标准库sync_Once源码分析/Rust并发：标准库sync_Once源码分析.html"><strong aria-hidden="true">7.3.</strong> Rust并发：标准库sync::Once源码分析</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Rust源码阅读：引用计数Rc/Rust源码阅读：引用计数Rc.html"><strong aria-hidden="true">7.4.</strong> Rust源码阅读：引用计数Rc</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Rust源码阅读：Cell、RefCell与内部可变性/Rust源码阅读：Cell、RefCell与内部可变性.html"><strong aria-hidden="true">7.5.</strong> Rust源码阅读： Cell、RefCell与内部可变性</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/关于_Rust_的_UnsafeCell、Cell_与_RefCell/关于_Rust_的_UnsafeCell、Cell_与_RefCell.html"><strong aria-hidden="true">7.6.</strong> 关于 Rust 的 UnsafeCell、Cell 与 RefCell</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Rust_Async_async-stream源码分析/Rust_Async_async-stream源码分析.html"><strong aria-hidden="true">7.7.</strong> Rust Async: async-stream源码分析</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/走进Tokio的异步世界/走进Tokio的异步世界.html"><strong aria-hidden="true">7.8.</strong> 走进 Tokio 的异步世界</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/tokio.rs_runtime的实现/tokio.rs_runtime的实现.html"><strong aria-hidden="true">7.9.</strong> tokio.rs runtime 的实现</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Tokio_internals/Tokio_internals.html"><strong aria-hidden="true">7.10.</strong> Tokio internals</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Tokio_internals/译文：Tokio內部机制.html"><strong aria-hidden="true">7.11.</strong> 译：Tokio 内部机制</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Rust_Axum_HTTP_框架的架构分析/Rust_Axum_HTTP_框架的架构分析.html"><strong aria-hidden="true">7.12.</strong> Rust Axum HTTP 框架的架构分析</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/安利一个Rust_Game_Engine：Bevy--ECS部分/安利一个Rust_Game_Engine：Bevy--ECS部分.html"><strong aria-hidden="true">7.13.</strong> 安利一个Rust Game Engine：Bevy--ECS部分</a></li><li class="chapter-item expanded "><a href="../../09_源码分析/Tokio_解析之任务调度/Tokio_解析之任务调度.html"><strong aria-hidden="true">7.14.</strong> Tokio 解析之任务调度</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">8.</strong> 生态观察</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../11_生态观察/Rust_web_frameworks_have_subpar_error_reporting/Rust_web_frameworks_have_subpar_error_reporting.html"><strong aria-hidden="true">8.1.</strong> Rust web frameworks have subpar error reporting</a></li><li class="chapter-item expanded "><a href="../../11_生态观察/SeaORM：要做Rust版本的ActiveRecord/SeaORM：要做Rust版本的ActiveRecord.html"><strong aria-hidden="true">8.2.</strong> SeaORM：要做Rust版本的ActiveRecord</a></li></ol></li><li class="chapter-item expanded "><a href="../../empty.html"><strong aria-hidden="true">9.</strong> 死灵终极</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../12_死灵终极/Learn_Rust_the_Dangerous_Way_系列文章翻译/Learn_Rust_the_Dangerous_Way_系列文章翻译_总述.html"><strong aria-hidden="true">9.1.</strong> 译：Learn Rust the Dangerous Way 总述</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Learn_Rust_the_Dangerous_Way_系列文章翻译/Learn_Rust_the_Dangerous_Way_系列文章翻译_0.html"><strong aria-hidden="true">9.2.</strong> 译：Learn Rust the Dangerous Way 0</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Learn_Rust_the_Dangerous_Way_系列文章翻译/Learn_Rust_the_Dangerous_Way_系列文章翻译_1.html"><strong aria-hidden="true">9.3.</strong> 译：Learn Rust the Dangerous Way 1</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Learn_Rust_the_Dangerous_Way_系列文章翻译/Learn_Rust_the_Dangerous_Way_系列文章翻译_2.html"><strong aria-hidden="true">9.4.</strong> 译：Learn Rust the Dangerous Way 2</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Learn_Rust_the_Dangerous_Way_系列文章翻译/Learn_Rust_the_Dangerous_Way_系列文章翻译_3.html"><strong aria-hidden="true">9.5.</strong> 译：Learn Rust the Dangerous Way 3</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Learn_Rust_the_Dangerous_Way_系列文章翻译/Learn_Rust_the_Dangerous_Way_系列文章翻译_4.html"><strong aria-hidden="true">9.6.</strong> 译：Learn Rust the Dangerous Way 4</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Learn_Rust_the_Dangerous_Way_系列文章翻译/Learn_Rust_the_Dangerous_Way_系列文章翻译_5.html"><strong aria-hidden="true">9.7.</strong> 译：Learn Rust the Dangerous Way 5</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Unsafe_Rust_随堂小测/Unsafe_Rust_随堂小测（一）.html"><strong aria-hidden="true">9.8.</strong> Unsafe Rust 随堂小测（一）</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Unsafe_Rust_随堂小测/Unsafe_Rust_随堂小测（二）.html"><strong aria-hidden="true">9.9.</strong> Unsafe Rust 随堂小测（二）</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Unsafe_Rust_随堂小测/Unsafe_Rust_随堂小测（三）.html"><strong aria-hidden="true">9.10.</strong> Unsafe Rust 随堂小测（三）</a></li><li class="chapter-item expanded "><a href="../../12_死灵终极/Unsafe_Rust_随堂小测/Unsafe_Rust_随堂小测参考答案.html"><strong aria-hidden="true">9.11.</strong> Unsafe Rust 随堂小测参考答案</a></li></ol></li></ol>
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                        <h1 id="数据库表达式执行的黑魔法用-rust-做类型体操之用宏展开重复代码-part-3--4"><a class="header" href="#数据库表达式执行的黑魔法用-rust-做类型体操之用宏展开重复代码-part-3--4">数据库表达式执行的黑魔法：用 Rust 做类型体操之用宏展开重复代码 (Part 3 &amp; 4)</a></h1>
<p>作者：<a href="https://www.zhihu.com/people/skyzh">迟策</a></p>
<p>原载：<a href="https://zhuanlan.zhihu.com/p/461657165">https://zhuanlan.zhihu.com/p/461657165</a></p>
<p>上篇：<a href="https://zhuanlan.zhihu.com/p/461405621">数据库表达式执行的黑魔法：用 Rust 做类型体操 (Part 2)</a></p>
<p>下篇：<a href="https://zhuanlan.zhihu.com/p/461901665">数据库表达式执行的黑魔法：与 Rust 编译器斗智斗勇 (Part 5 &amp; 6)</a></p>
<h2 id="day-3-实现-arrayimpl-等动态调度类型"><a class="header" href="#day-3-实现-arrayimpl-等动态调度类型">Day 3: 实现 <code>ArrayImpl</code> 等动态调度类型</a></h2>
<p>在 Day 3-4 中，我们要考虑一件事情：数据库系统在编译期是不可能知道每个从存储、从网络上传过来的 Array 是什么类型的。我们需要给 Array 提供动态分发的功能。</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn eval_binary(i1: Box&lt;dyn Array&gt; i2: Box&lt;dyn Array&gt;) -&gt; Box&lt;dyn Array&gt; {
  /* some black magic */
}
<span class="boring">}
</span></code></pre></pre>
<p>说到动态分发，同学们一定会立刻想到用 <code>Box&lt;dyn Array&gt;</code> 来表示 Array 的动态类型。可惜的是，<code>Array</code>trait 在历经 Day 0, 1, 2 的类型体操之后，已经不符合 object safety 了。</p>
<p>如果一个 trait 可以被包裹成一个动态分发的对象（如 <code>Box&lt;dyn Array&gt;</code>），编译器会给 <code>dyn Array</code> 实现 <code>Array</code> trait。我们看看现在的 <code>Array</code> trait 长啥样：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>/// [`Array`] is a collection of data of the same type.
pub trait Array /* 省略一些 bound */ {
    /// Retrieve a reference to value.
    fn get(&amp;self, idx: usize) -&gt; Option&lt;Self::RefItem&lt;'_&gt;&gt;;

    /// Number of items of array.
    fn len(&amp;self) -&gt; usize;

    /// Indicates whether this array is empty
    fn is_empty(&amp;self) -&gt; bool {
        self.len() == 0
    }

    /// Get iterator of this array.
    fn iter(&amp;self) -&gt; ArrayIterator&lt;Self&gt;;
}
<span class="boring">}
</span></code></pre></pre>
<p>对于 <code>Int32Array</code>，<code>get</code> 返回 <code>Option&lt;i32&gt;</code>；而对于 <code>StringArray</code>, <code>get</code> 返回 <code>Option&lt;&amp;str&gt;</code>。因此，这个 trait 是不可能给 <code>dyn Array</code> 实现的——根本没有统一的签名能够实现。</p>
<p>因此，我们考虑使用 <code>enum</code> 来做 dispatch：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub enum ArrayImpl {
    Int32(I32Array),
    Float32(F32Array),
    String(StringArray),
}
<span class="boring">}
</span></code></pre></pre>
<p>这时同学们一定有想问：既然是 enum，为啥不用 <a href="https://link.zhihu.com/?target=https%3A//crates.io/crates/enum_dispatch">enum_dispatch</a> 这个库捏？</p>
<p>enum_dispatch 也会自动为 ArrayImpl 这个 enum 类型实现 <code>Array</code> trait，但上面的一通分析表明这是不可能的，所以我们要自己亲自动手实现。</p>
<h3 id="arrayimpl-的函数签名"><a class="header" href="#arrayimpl-的函数签名">ArrayImpl 的函数签名</a></h3>
<p><code>ArrayImpl</code> 实现的 <code>get</code> 函数，究竟用什么签名比较好呢？</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl ArrayImpl {
  fn get(&amp;self, idx: usize) -&gt; Option&lt;/* ??? */&gt;;
}
<span class="boring">}
</span></code></pre></pre>
<p>考虑到这里只能填单个类型，我们把 <code>Scalar</code>, <code>ScalarRef</code> 也做一整套 enum 出来：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>/// Encapsules all variants of [`Scalar`]
pub enum ScalarImpl {
    Int32(i32),
    Float32(f32),
    String(String),
}

/// Encapsules all variants of [`ScalarRef`]
pub enum ScalarRefImpl&lt;'a&gt; {
    Int32(i32),
    Float32(f32),
    String(&amp;'a str),
}
<span class="boring">}
</span></code></pre></pre>
<p>这样，<code>ArrayImpl</code> 就可以用这个签名了：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl ArrayImpl {
  fn get(&amp;self, idx: usize) -&gt; Option&lt;ScalarRefImpl&lt;'_&gt;&gt;;
}
<span class="boring">}
</span></code></pre></pre>
<h3 id="实现-tryfrom-和-into-trait"><a class="header" href="#实现-tryfrom-和-into-trait">实现 <code>TryFrom</code> 和 <code>Into</code> trait</a></h3>
<p>接下来的目标就是在泛型函数中将 <code>ArrayImpl</code> 转换成一个 generic 类型。</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn eval_binary&lt;I1: Array, I2: Array&gt;(i1: &amp;ArrayImpl, i2: &amp;ArrayImpl) -&gt; Result&lt;ArrayImpl&gt; {
    let i1: &amp;I1 = i1.try_into()?;
    let i2: &amp;I2 = i2.try_into()?;
    /* some black magic */
}
<span class="boring">}
</span></code></pre></pre>
<p>这就要求我们给 <code>Array</code> 加上 <code>TryFrom</code> 和 <code>Into</code> 的 bound。</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub trait Array: Send + Sync + Sized + 'static + TryFrom&lt;ArrayImpl&gt; + Into&lt;ArrayImpl&gt;
where
    for&lt;'a&gt; Self::OwnedItem: Scalar&lt;RefType&lt;'a&gt; = Self::RefItem&lt;'a&gt;&gt;,
{
<span class="boring">}
</span></code></pre></pre>
<p>简单实现一下各个 <code>Array</code> 的 <code>TryFrom</code> 和 <code>ArrayImpl</code> 的 <code>From</code>，带上这个 bound 就可以编译通过了。实现的方式就是 match <code>ArrayImpl</code> 的 enum variant，然后分别做 dispatch。这里又会碰到 <code>PrimitiveArray</code> blanket implementation 导致的类型不匹配的坑，记得要给 <code>PrimitiveArray</code> 多加两个 bound: <code>Into&lt;ArrayImpl&gt;</code> 和 <code>TryFrom&lt;ArrayImpl&gt;</code></p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl&lt;T&gt; Array for PrimitiveArray&lt;T&gt;
where
    T: PrimitiveType,
    T: Scalar&lt;ArrayType = Self&gt;,
    for&lt;'a&gt; T: ScalarRef&lt;'a, ScalarType = T, ArrayType = Self&gt;,
    for&lt;'a&gt; T: Scalar&lt;RefType&lt;'a&gt; = T&gt;,
    Self: Into&lt;ArrayImpl&gt;,
    Self: TryFrom&lt;ArrayImpl&gt;,
{
<span class="boring">}
</span></code></pre></pre>
<h3 id="表达-arrayimpl-引用的-tryfrom-bound"><a class="header" href="#表达-arrayimpl-引用的-tryfrom-bound">表达 <code>ArrayImpl</code> 引用的 <code>TryFrom</code> bound</a></h3>
<p>回顾一下之前 <code>eval_binary</code> 的签名：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn eval_binary&lt;I1: Array, I2: Array&gt;(i1: &amp;ArrayImpl, i2: &amp;ArrayImpl) -&gt; Result&lt;ArrayImpl&gt; {
    let i1: &amp;I1 = i1.try_into()?;
    let i2: &amp;I2 = i2.try_into()?;
    /* some black magic */
}
<span class="boring">}
</span></code></pre></pre>
<p>等等，好像有点不太对：这里要求的是 <code>&amp;Array: TryFrom&lt;&amp;ArrayImpl&gt;</code>，而不是 <code>Array: TryFrom&lt;ArrayImpl&gt;</code>。如何表达 <code>Array</code> 的引用有 <code>TryFrom</code> 的性质呢？</p>
<p>再次结合之前 Day 2 的经验，我们可以用 <a href="https://link.zhihu.com/?target=https%3A//doc.rust-lang.org/nomicon/hrtb.html">HRTB</a> 来表示这个性质：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub trait Array: Send + Sync + Sized + 'static + TryFrom&lt;ArrayImpl&gt; + Into&lt;ArrayImpl&gt;
where
    for&lt;'a&gt; Self::OwnedItem: Scalar&lt;RefType&lt;'a&gt; = Self::RefItem&lt;'a&gt;&gt;,
    for&lt;'a&gt; &amp;'a Self: TryFrom&lt;&amp;'a ArrayImpl&gt;
{
<span class="boring">}
</span></code></pre></pre>
<p>加好以后，诶，为什么编译过不了了？编译器报了 15 个错，都是在使用 <code>Array</code> 的地方：</p>
<pre><code class="language-text">error[E0277]: the trait bound `for&lt;'a&gt; &amp;'a A: From&lt;&amp;'a array::ArrayImpl&gt;` is not satisfied
   --&gt; archive/day3/src/array/iterator.rs:8:33
    |
8   | pub struct ArrayIterator&lt;'a, A: Array&gt; {
    |                                 ^^^^^ the trait `for&lt;'a&gt; From&lt;&amp;'a array::ArrayImpl&gt;` is not implemented for `&amp;'a A`
    |
    = note: required because of the requirements on the impl of `for&lt;'a&gt; Into&lt;&amp;'a A&gt;` for `&amp;'a array::ArrayImpl`
note: required because of the requirements on the impl of `for&lt;'a&gt; TryFrom&lt;&amp;'a array::ArrayImpl&gt;` for `&amp;'a A`
   --&gt; archive/day3/src/scalar.rs:185:10
    |
185 | impl&lt;'a&gt; TryFrom&lt;ScalarRefImpl&lt;'a&gt;&gt; for &amp;'a str {
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^     ^^^^^^^
note: required by a bound in `array::Array`
   --&gt; archive/day3/src/array.rs:24:23
    |
21  | pub trait Array: Send + Sync + Sized + 'static + TryFrom&lt;ArrayImpl&gt; + Into&lt;ArrayImpl&gt;
    |           ----- required by a bound in this
...
24  |     for&lt;'a&gt; &amp;'a Self: TryFrom&lt;&amp;'a ArrayImpl&gt;
    |                       ^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `array::Array`
help: consider introducing a `where` bound, but there might be an alternative better way to express this requirement
    |
8   | pub struct ArrayIterator&lt;'a, A: Array&gt; where &amp;'a A: for&lt;'a&gt; From&lt;&amp;'a array::ArrayImpl&gt; {
    |                                        +++++++++++++++++++++++++++++++++++++++++++++++
</code></pre>
<p>给 <code>Array</code> 加上这个 bound，会导致我们需要在所有用到 <code>Array</code> 的地方都写上 <code>where &amp;'a A: for&lt;'a&gt; From&lt;&amp;'a array::ArrayImpl&gt;</code>。这样就给使用者造成了很大的负担：开发者明明只想用 <code>Array</code> 来写一些泛型 SQL scalar 函数，结果每次写的时候都要带上一大堆 trait bound，这简直是徒增开发难度！</p>
<p>怎么办呢？考虑到几个点：</p>
<ul>
<li>开发者一般只需要实现原始类型的泛型函数。</li>
<li>运行时的类型转换由向量化框架来做。</li>
</ul>
<p>这么一想，那其实在我们表达式框架的内部要求这个 bound 是最好的解决方案了。</p>
<p>去掉 <code>Array</code> 上的 <code>TryFrom</code> bound，把它写在 <code>eval_binary</code> 上：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn eval_binary&lt;'a, I1: Array, I2: Array&gt;(
    i1: &amp;'a ArrayImpl,
    i2: &amp;'a ArrayImpl,
) -&gt; Result&lt;ArrayImpl, ()&gt;
where
    &amp;'a I1: TryFrom&lt;&amp;'a ArrayImpl, Error = ()&gt;,
    &amp;'a I2: TryFrom&lt;&amp;'a ArrayImpl, Error = ()&gt;,
{
    let i1: &amp;'a I1 = i1.try_into()?;
    let i2: &amp;'a I2 = i2.try_into()?;
    todo!()
}
<span class="boring">}
</span></code></pre></pre>
<p>然后，编译通过，Part 3 就结束了！</p>
<h2 id="day-4-macro-生成-boilerplate-code"><a class="header" href="#day-4-macro-生成-boilerplate-code">Day 4: Macro 生成 boilerplate code</a></h2>
<p>随着我们的 <code>Array</code> 类型越来越多，我们需要手写的代码也越来越多。比如 <code>ArrayImpl::get</code>：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl ArrayImpl {
    pub fn get(&amp;self, idx: usize) -&gt; Option&lt;ScalarRefImpl&lt;'_&gt;&gt; {
        match self {
            Self::Int32(a) =&gt; a.get(idx).map(ScalarRefImpl::Int32),
            Self::Int64(a) =&gt; a.get(idx).map(ScalarRefImpl::Int64),
            Self::Float32(a) =&gt; a.get(idx).map(ScalarRefImpl::Float32),
            // 每添加一个类型就要多写一个 arm ...
        }
    }
}
<span class="boring">}
</span></code></pre></pre>
<p>包括 <code>TryFrom</code>, <code>len</code> 等等函数，也需要我们手动实现动态分发。</p>
<p>有没有什么办法可以写一次就自动给所有类型生成相关的动态分发代码呢？聪明的你一定想到了用宏展开来解决：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>/// Implements dispatch functions for [`Array`]
macro_rules! impl_array_dispatch {
    ($( { $Abc:ident, $abc:ident, $AbcArray:ty, $AbcArrayBuilder:ty, $Owned:ty, $Ref:ty } ),*) =&gt; {
        impl ArrayImpl {
            /// Get the value at the given index.
            pub fn get(&amp;self, idx: usize) -&gt; Option&lt;ScalarRefImpl&lt;'_&gt;&gt; {
                match self {
                    $(
                        Self::$Abc(array) =&gt; array.get(idx).map(ScalarRefImpl::$Abc),
                    )*
                }
            }
        }
    }
}

impl_array_dispatch! {
    { Int32, int32, Int32Array, Int32ArrayBuilder, i32, i32 },
    { Int64, int64, Int64Array, Int64ArrayBuilder, i64, i64 },
    { String, string, StringArray, StringArrayBuilder, String, &amp;'a str },
    // ...
}
<span class="boring">}
</span></code></pre></pre>
<p>问题来了：我们的 <code>impl_array_dispatch</code> macro 肯定是到处都有——有的用来 impl <code>TryFrom</code> for <code>ArrayImpl</code>, 有的用来 impl <code>ArrayImpl</code>。如果我们有多个这样的 macro，macro 展开的参数 <code>{ Int32, int32, Int32Array, Int32ArrayBuilder, i32, i32 }</code> 也要写很多份。</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl_array_dispatch! {
    { Int32, int32, Int32Array, Int32ArrayBuilder, i32, i32 },
    { Int64, int64, Int64Array, Int64ArrayBuilder, i64, i64 },
    { String, string, StringArray, StringArrayBuilder, String, &amp;'a str },
    // ...
}

impl_array_try_from! {
    { Int32, int32, Int32Array, Int32ArrayBuilder, i32, i32 },
    { Int64, int64, Int64Array, Int64ArrayBuilder, i64, i64 },
    { String, string, StringArray, StringArrayBuilder, String, &amp;'a str },
    // ...
}

// ...
<span class="boring">}
</span></code></pre></pre>
<p>有什么办法可以解决这个问题呢？试试 macro 能不能传 macro 参数进去：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>macro_rules! list_all_variants! {
    () =&gt; {
        { Int16, int16, I16Array, I16ArrayBuilder, i16, i16 },
        { Int32, int32, I32Array, I32ArrayBuilder, i32, i32 },
        { Int64, int64, I64Array, I64ArrayBuilder, i64, i64 },
        { Float32, float32, F32Array, F32ArrayBuilder, f32, f32 },
        { Float64, float64, F64Array, F64ArrayBuilder, f64, f64 },
        { Bool, bool, BoolArray, BoolArrayBuilder, bool, bool },
        { String, string, StringArray, StringArrayBuilder, String, &amp;'a str }
    }
}

impl_array_dispatch! { list_all_variants! {} }
<span class="boring">}
</span></code></pre></pre>
<p>紧接着编译器报了一堆错，五彩缤纷：</p>
<p><img src="assets/003/0010.png" alt="" /></p>
<p>这是为什么捏？</p>
<p>这就要从 Rust macro 的执行方式讲起了。我们用正常的 Rust 程序来打个比方：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>fn list_all_variants() -&gt; Vec&lt;Variants&gt;;
fn impl_array_dispatch(data: Vec&lt;Variants&gt;) -&gt; GeneratedCode;

impl_array_dispatch(list_all_variants());
<span class="boring">}
</span></code></pre></pre>
<p>对于 Rust 程序来说，执行的方法是先调用 <code>list_all_variants()</code> 获得返回值，然后把这个返回值交给 <code>impl_array_dispatch</code> 来继续执行，由里到外展开。</p>
<p>对于 Rust macro 来说，情况就截然相反：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl_array_dispatch! { list_all_variants! {} }
<span class="boring">}
</span></code></pre></pre>
<p>在这里，Rust 编译时先尝试展开 <code>impl_array_dispatch</code> 这个 macro。此时，<code>list_all_variants! {}</code> 这些 token 会作为 macro 的参数传进去。但我们需要的是符合 <code>$( { $Abc:ident, $abc:ident, $AbcArray:ty, $AbcArrayBuilder:ty, $Owned:ty, $Ref:ty } ),*</code> 的一个参数，<code>list_all_variants! {}</code> 这四个 token 显然不满足 <code>impl_array_dispatch</code> macro 需要的参数。</p>
<p>怎么在这种由外到里展开的顺序里面实现宏代码的复用呢？我拍脑袋想出了一种聪明的办法：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>/// `for_all_variants` includes all variants of our array types. If you added a new array
/// type inside the project, be sure to add a variant here.
///
/// Every tuple has four elements, where
/// `{ enum variant name, function suffix name, array type, builder type, scalar type }`
macro_rules! for_all_variants {
    ($macro:tt) =&gt; {
        $macro! {
            { Int16, int16, I16Array, I16ArrayBuilder, i16, i16 },
            { Int32, int32, I32Array, I32ArrayBuilder, i32, i32 },
            { Int64, int64, I64Array, I64ArrayBuilder, i64, i64 },
            { Float32, float32, F32Array, F32ArrayBuilder, f32, f32 },
            { Float64, float64, F64Array, F64ArrayBuilder, f64, f64 },
            { Bool, bool, BoolArray, BoolArrayBuilder, bool, bool },
            { String, string, StringArray, StringArrayBuilder, String, &amp;'a str }
        }
    };
}

for_all_variants! { impl_array_builder_dispatch }
<span class="boring">}
</span></code></pre></pre>
<p>这样一来，Rust 先展开 <code>for_all_variants</code>，得到：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl_array_builder_dispatch! {
    { Int16, int16, I16Array, I16ArrayBuilder, i16, i16 },
    { Int32, int32, I32Array, I32ArrayBuilder, i32, i32 },
    { Int64, int64, I64Array, I64ArrayBuilder, i64, i64 },
    { Float32, float32, F32Array, F32ArrayBuilder, f32, f32 },
    { Float64, float64, F64Array, F64ArrayBuilder, f64, f64 },
    { Bool, bool, BoolArray, BoolArrayBuilder, bool, bool },
    { String, string, StringArray, StringArrayBuilder, String, &amp;'a str }
}
<span class="boring">}
</span></code></pre></pre>
<p>这和我们在最开始手敲这些类型声明时的 macro 代码一摸一样。</p>
<p>所以，通过调换 <code>impl_array_builder_dispatch</code> 和模版 <code>for_all_variants</code> 的内外位置关系，我们就做到了 <code>for_all_variants</code> 内部信息的复用。这样以来，我们就可以到处实现动态分发的函数了：</p>
<pre><pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>for_all_variants! { impl_array_builder_dispatch }
for_all_variants! { impl_array_try_from }
for_all_variants! { impl_scalar_try_from }
// ...
<span class="boring">}
</span></code></pre></pre>
<p>于是乎，我们的 <code>ArrayImpl</code> 可以在修改少量代码（往 <code>for_all_variants</code> 里面加一条记录）的情况下支持越来越多的类型。Day 4 也结束了！</p>
<h3 id="思考题"><a class="header" href="#思考题">思考题</a></h3>
<p>在本系列中，我们通过 <code>ArrayImpl</code> 来做动态分发。是否可以用 <code>pub struct BoxedArray(Box&lt;dyn Any&gt;)</code> 来做动态分发？</p>
<p>答案是可以，我们可以有很多种做法：</p>
<ul>
<li>使用 <code>Any</code> 的 <code>TypeId</code> 替代 <code>ArrayImpl::XXX</code> 来判断应该 downcast 到什么类型，然后分发。</li>
<li>为 Array 实现一套新的 trait：<code>ArrayDispatch</code>，里面加一个 <code>array_type(&amp;self) -&gt; ArrayTypeEnum</code> 函数。定义 <code>pub struct BoxedArray(Box&lt;dyn ArrayDispatch&gt;)</code>，通过 <code>array_type</code> 来决定 downcast 到什么类型。</li>
</ul>
<p>上篇：<a href="https://zhuanlan.zhihu.com/p/461405621">数据库表达式执行的黑魔法：用 Rust 做类型体操 (Part 2)</a></p>
<p>下篇：<a href="https://zhuanlan.zhihu.com/p/461901665">数据库表达式执行的黑魔法：与 Rust 编译器斗智斗勇 (Part 5 &amp; 6)</a></p>
<p>您可以在 <a href="https://link.zhihu.com/?target=https%3A//github.com/skyzh/type-exercise-in-rust">type-exercise-in-rust</a> 中围观整个系统的实现。</p>
<p><em>（系列文章持续更新中，下一次更新时间看心情）</em></p>
<p>编辑于 2022-02-02 16:14</p>

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